The Spiral nebulæ are very curious and complicated objects, but they are visible only in the largest telescopes. Prominent above all is the double spiral nebula No. 3,572, in Canes Venatici, which is not far from η Ursa Majoris. In Lord Rosse's telescope, this object presents a wonderful spiral disposition, looking somewhat like one of the fire-works called pin-wheels, and forming long, curved wisps, diverging from two bright centres. The spectrum of this object, however, is not that of a gas. In the constellation Virgo, Rosse has detected another such nebula. In Cepheus, Triangulum, and Ursa Major, are found other spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral nebulæ and suspected a similar structure in 30 others.
The class of the Irregular nebulæ, which will be now considered, differs greatly in character from the others, and includes the largest, the brightest and the most extraordinary nebulæ in the heavens. The nebulæ of this class differ from those belonging to the other classes by a want of symmetry in their form and in the distribution of their light, as well as by their capricious shapes, and their very complicated structure. Another and perhaps the principal difference between them and the objects above described, consists in the remarkable fact already stated, that they are not, except in rare cases, to be found in the regions where the other nebulæ abound. On the contrary, they are found in or very near the Milky-way, precisely where the other nebulæ are the most rare. This fact, recognized by Sir J. Herschel, led him to consider them as "outlying, very distant, and as it were detached fragments of the great stratum of the Galaxy." It seems very probable that the reason why these objects differ so greatly from the other nebulæ in size, brightness and complication of structure, is simply because they are much nearer to us than are most of the others. They are perhaps nebulous members of our Galaxy. The same remark which has been made of star-clusters may be applied to nebulæ. The nearer they are to us, the larger, the brighter and the more complicated they will appear, while the farther they are removed, the more simple and regular and round they will appear, only their brightest and deepest parts being then visible.
The Crab Nebula of Lord Rosse, near ζ Tauri, No. 1,157, is one of the interesting objects of this class. It has curious appendages streaming off from an oval, luminous mass, which give it a distant resemblance to the animal from which it derives its name. The Bifid nebula in Cygnus, Nos. 4,400 and 4,616, is another object of this class. It consists of a long, narrow, crooked streak, forking out at several places, and passing through χ Cygni. Observers, having failed to recognize the connection existing between its different centres of brightness, have made distinct nebulæ of this extended object.
The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious object, with a general resemblance to the instrument from which it derives its name. Lord Rosse's telescope has shown many stars in it, projected on a nebulous background, and Prof. Bond seems to have thought that it showed traces of resolvability, although in the study which I made of this nebula with the same instrument used by the latter observer, I failed to perceive any such traces. Dr. Huggins finds its spectrum gaseous.
The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by Sir J. Herschel as a loose cluster of at least 100 stars, I have found to be involved in an extensive, although not very bright, nebula, which would seem to have escaped his scrutiny. In a study and drawing of this nebula made in 1876, its general form is that of an open fan, with the exception that the handle is wanting, with deeply indented branches on the preceding side, where the brightest stars of the cluster are grouped. From its peculiar form, this object might appropriately be called the Fan nebula.
The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I have made a study and two drawings, one with a refractor 6⅓ inches in aperture, and the other conjointly with Prof. Holden, with the great telescope of the Naval Observatory, is a bright and very complicated object. Its general appearance in small instruments, with low power, is that of a long, narrow pisciform mass of light, from which proceeds on the preceding side, the great double loop from which it derives its name. But in the great Washington refractor its structure becomes very complicated, forming various bright nebulous masses and wisps of great extension. Prof. Holden, who has made a careful, comparative study of the published drawings of this object, thinks there are reasons to believe that its western branch has moved relatively to the stars found within its loop. The spectrum of this nebula is gaseous.
The Trifid nebula, No. 4,355, in the same constellation, is also a very remarkable object, although it is not so bright as the last. This nebula, which I have studied with the refractor of the Cambridge Observatory, consists of four principal masses of light, separated by a wide and irregular gap branching out in several places. These masses, which are brighter along the dark gap, gradually fade away externally. A group of stars, two of which are quite bright, is found near the centre of the nebula, on the inner edge of the following mass, and close to the principal branch of the dark channel. A little to the north, and apparently forming a part of this nebula, is a globular-looking nebula, having a pale yellow star at its centre. Prof. Holden's studies on this nebula show that the triple star, which was centrally situated in the dark gap from 1784 to 1833, was found involved in the border of the nebulous mass following it, from 1839 to 1877; the change, he thinks, is attributable either to the proper motion of the group of stars or to that of the nebula itself.
In the same vicinity is found the splendid and very extensive nebula No. 4,361, in which is involved a loose, but very brilliant star-cluster. This nebula and cluster, which I have studied and drawn with a 6⅓ inch telescope, is very complicated in structure, and divided by a dark irregular gap into three principal masses of light, condensing at one point around a star, and at others forming long, bright, gently-curved branches, which give to this object a strong resemblance to the wings of a bird when extended upwards in the action of flying. From this peculiarity this object might appropriately be called the Winged nebula. Its spectrum is that of a gas.
The variable star η Argus is completely surrounded by the great nebula of the same name, No. 2,197, first delineated by Sir J. Herschel, during his residence at the Cape of Good Hope, in 1838. This object, which covers more than ⁴⁄₇ of a square degree, is divided into three unequal masses, separated by dark oval spots, comparatively free from nebulosity, and is suspected to have undergone changes since Herschel's time.
In the same field with the double star, ζ Orionis, the most easterly of the three bright stars in the belt of Orion, is found another irregular nebula of the Trifid type. From the drawings which I have made of this object, it appears to be composed of three principal unequal masses, separated by a wide, irregular, dark channel, two of the masses being quite complicated in structure, and forming curved, nebulous streams of considerable length and breadth. This nebula, like the next to be described, seems to be connected with the Galaxy by the great galactic loop described in another section.